In general, an organic light-emitting element has such a configuration that a thin film including an organic light-emitting material is formed on a substrate as its simplest structure. The organic light-emitting element is subjected to optical and electrical excitation so as to emit light. As a result, light is emitted from the organic light-emitting element. A photo luminescence element (PL element), in which optical excitation is performed, serves as the organic light-emitting element as long as at least the above-mentioned element configuration is satisfied. That is, the PL element utilizes a phenomenon in which an organic light-emitting material is excited by being irradiated with light depending on a light absorption wavelength of the organic light-emitting material, and light is then emitted as energy upon return from a conduction band to a valence band. On the other hand, an organic electroluminescence element (hereinafter, referred to as “organic EL element”), in which electrical excitation is performed, includes a light-emitting layer and a pair of counter electrodes sandwiching the light-emitting layer therebetween as its simplest structure. That is, the organic EL element utilizes a phenomenon in which electrons are injected from an cathode and holes are injected from an anode when an electric field is applied between both the electrodes, and light is emitted as energy upon return from a conduction band to a valence band of an energy level at which the electrons and the holes recombine with each other in a light-emitting layer.
In recent years, in particular, the organic EL element has increasingly been expected to find practical applications in energy-saving displays and lighting. In such circumstances, an organic EL element using an organic thin film has been developed actively. As a fluorescent organic compound to be used as a material for such organic EL element, there are known, for example, perylene, a thiazole derivative, a quinacridone derivative, rubrene, a benzophenone derivative, and a coumarin derivative. However, a conventional fluorescent organic compound involves the following fundamental problem in terms of excitation efficiency of the compound. That is, upon recombination of electrons and holes in a light-emitting layer of an organic EL element, singlet excitons as fluorescence-emitting excitons are formed only at a ratio of 25% of all excitons, yielding an internal quantum efficiency of 25% at the highest and a luminous efficiency of the organic EL element of about 5% at the highest (Non Patent Literature 1).
It has recently been found that when a specific porphyrin-based metal complex as a fluorescent organic compound is used as the light-emitting material for the organic EL element, the porphyrin-based metal complex emits thermally activated delayed fluorescence, leading to an improvement in exciton generation efficiency of the element (Patent Literatures 1 and 2 and Non Patent Literature 1).
Patent Literatures 1 and 2 disclose the following matters. In an organic EL element, carriers are injected from each of both electrodes, i.e., positive and negative electrodes to a light-emitting substance to generate a light-emitting substance in an excited state so as to emit light. It is generally said that in the case of a carrier injection type organic EL element, 25% of generated excitons are excited to an excited singlet state and the remaining 75% are excited to an excited triplet state. Accordingly, it is conceivable that utilization of light to be emitted from the excited triplet state, i.e., phosphorescence should provide higher energy use efficiency. However, in the phosphorescence, the excited triplet state has a long lifetime, and hence deactivation of energy occurs through saturation of an excited state and interactions with excitons in an excited triplet state, with the result that a high quantum yield is not obtained in many cases in general. In view of the foregoing, an organic EL element utilizing a material which emits delayed fluorescence is conceivable. A certain kind of fluorescent substance emits fluorescence via intersystem crossing or the like leading to energy transition to an excited triplet state and the subsequent reverse intersystem crossing to an excited singlet state through triplet-triplet annihilation or thermal energy absorption. In the organic EL element, it is considered that the latter material which emits thermally activated delayed fluorescence is particularly useful. In this case, when a delayed fluorescent material is utilized in the organic EL element, excitons in an excited singlet state emit fluorescence as per normal. On the other hand, excitons in an excited triplet state absorb heat produced from a device and undergo intersystem crossing to an excited singlet to emit fluorescence. The fluorescence in this case is light emission from the excited singlet and hence is light emission at the same wavelength as fluorescence. However, the fluorescence has a longer lifetime of light to be emitted, i.e., a longer emission lifetime than those of normal fluorescence and phosphorescence by virtue of reverse intersystem crossing from an excited triplet state to an excited singlet state, and hence is observed as fluorescence delayed as compared to the normal fluorescence and phosphorescence. This can be defined as delayed fluorescence. Through the use of such thermally activated type exciton transfer mechanism, i.e., through thermal energy absorption after carrier injection, the ratio of a compound in an excited singlet state, which has usually been generated only at a ratio of 25%, can be increased to 25% or more. The use of a compound which emits intense fluorescence and delayed fluoresce even at a low temperature of less than 100° C. results in sufficient intersystem crossing from an excited triplet state to an excited singlet state by means of heat of an device, contributing to emission of delayed fluorescence. Thus, the luminous efficiency is drastically improved.
Based on such hypothesis, Patent Literatures 1 and 2 and Non Patent Literature 1 each disclose that a specific porphyrin-based metal complex emits delayed fluorescence. However, none of the literatures discloses a relationship between the luminous efficiency and a difference between excited singlet energy and excited triplet energy and has any description suggesting the possibility of delayed fluorescence in an organic compound containing no metal atom other than the porphyrin-based metal complex. Further, the organic EL element according to each of the reports provides significantly lower luminous efficiency than a theoretical value. Thus, it is desired that an additional improvement be made in order to use the element in actual applications such as a display, a display element, a backlight, and lighting.
Patent Literatures 3 and 4 each disclose that a compound having an indolocarbazole skeleton is used in an organic EL element. However, none of the literatures discloses that delayed fluorescence is emitted in light emission of the compound itself.